HCS515 View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

HCS515

KEELOQ Code Hopping Decoder

Microchip Technology 

HCS515

1.0 KEELOQ SYSTEM OVERVIEW

1.1 Key Terms

• Manufacturer’s Code – A 64-bit word, unique to

each manufacturer, used to produce a unique

encoder decryption key in each transmitter.

• Encoder Decryption Key – A 64-bit key, unique for

each transmitter. The encoder decryption key

controls the KEELOQ decryption algorithm and is

stored in EEPROM on the decoder device.

• Learn – The receiver uses information that is

transmitted to derive the transmitter’s encoder

decryption key, decrypt the discrimination value,

and the synchronization counter in learning mode.

The encoder decryption key is a function of the

manufacturer’s code and the device serial number

and/or seed value.

The HCS encoders and decoders employ the KEELOQ

code hopping technology and a KEELOQ encryption

algorithm to achieve a high level of security. Code

hopping is a method by which the code transmitted

from the transmitter to the receiver is different every

time a button is pushed. This method, coupled with a

transmission length of 66 bits, virtually eliminates the

use of code ‘grabbing’ or code ‘scanning’.

1.2 HCS Encoder Overview

The HCS encoders have a small EEPROM array which

must be loaded with several parameters before use.

The most important of these values are:

• An encoder decryption key that is generated at

the time of production

• A 16-bit synchronization counter value

• A 28-bit serial number which is meant to be

unique for every encoder

The manufacturer programs the serial number for each

encoder at the time of production, while the ‘Key Gen-

eration Algorithm’ generates the encoder decryption

key (Figure 1-1). Inputs to the key generation algorithm

typically consist of the encoder’s serial number and a

64-bit manufacturer’s code, which the manufacturer

creates.

Note:

The manufacturer code is a pivotal part of

the system’s overall security. Conse-

quently, all possible precautions must be

taken and maintained for this code.

The 16-bit synchronization counter is the basis for the

transmitted code changing for each transmission and is

updated each time a button is pressed. Because of the

complexity of the KEELOQ encryption algorithm, a

change in one bit of the synchronization counter value

will result in a large change in the actual transmitted

code. There is a relationship (Figure 1-2) between the

encoder decryption key values in EEPROM and how

they are used in the encoder. Once the encoder

detects that a button has been pressed, the encoder

reads the button and updates the synchronization

counter. The synchronization value is then combined

with the encoder decryption key in the KEELOQ

encryption algorithm, and the output is 32 bits of

encrypted information. This data will change with every

button press, hence, it is referred to as the code hop-

ping portion of the code word. The 32-bit code hopping

portion is combined with the button information and the

serial number to form the code word transmitted to the

receiver.

FIGURE 1-1: CREATION AND STORAGE OF ENCRYPTION KEY DURING PRODUCTION

Transmitter

Serial Number or

Seed

Manufacturer’s

Code

Key

Generation

Algorithm

Encryption

Key

HCS515 EEPROM Array

Serial Number

Encryption Key

Sync Counter

.

.

.

DS40183A-page 2

Preliminary

© 1998 Microchip Technology Inc.

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